In a perpendicular magnetic recording system in which a magnetic recording medium is magnetized in the direction of thickness, it is recognized that the shorter the wavelength of a magnetization which is magnetically recorded, the less in principle will be the self-demagnetization within the recording medium, a result that is suitable in achieving a high density recording (refer Nikkei Electronics, Oct. 25, 1982 Issue, pages 142 to 143).
A perpendicular magnetic recording requires a recording medium which exhibits a pronounced magnetic anisotropy in the perpendicular direction, and a magnetic recording head which develops a strong, sharply defined perpendicular component field. Known magnetic recording mediums which exhibit such a pronounced magnetic anisotropy in the perpendicular direction includes Co-Cr, barium ferrite, Co-O and the like. A magnetically soft film of a high magnetic permeability comprising Fe-Ni, permalloy, etc., is formed on a flexible base which may be formed by polyethylene terephthalate (PET) or the like, followed by Co-Cr film which is formed thereon, thus providing a two layer perpendicular magnetic recording medium which is suitable for a high density recording.
For use as a magnetic head which develops a strong, sharply defined perpendicular component field, there is proposed an auxiliary pole excited magnetic head (refer S. Iwasaki and Y. Nakamura; "An Analysis for the Magnetization Mode for High Density Magnetic Recording"; IEEE Trans. on Magn., Vol. MAG-13, No. 5, p 1272-1277, September 1977).
Such an auxiliary pole excited magnetic head 10 is illustrated in FIG. 1. As shown, it comprises a main pole 1, and an auxiliary pole 2 which is disposed in opposing relationship with the main pole 1 and is spaced therefrom by a given air gap 3, the auxiliary pole comprising a block 2a of a high permeability magnetic material such as ferrite and carrying a winding 2b thereon which is utilized to pass or develop an electric current representing information signal. During a recording and reproduction, a magnetic recording medium 4 is placed into the air gap 3 with the magnetized or magnetizable film of the medium 4 placed in sliding contact with the lower end face of the main pole 1. As shown in FIG. 2, the main pole 1 may comprise a film 1a of a magnetically soft material having a high permeability such as Co-Zr-Nb (hereafter referred to as a main pole film) which is held sandwiched between a pair of joined members such as a magnetic member 1b as may be formed from ferrite and a non-magnetic member 1c as may be formed from glass. Alternatively, a main pole 1' shown in FIG. 3 comprises the main pole film 1a which is held sandwiched between a joined member including the magnetic member 1b and the non-magnetic member 1c located on one side and a non-magnetic member 1c' which is constructed of the same material as that used to form the non-magnetic member 1c.
The magnetic recording and reproduction onto or from the magnetic recording medium 4 with the described head 10 takes place as follows: Initially, when effecting a magnetic recording, an electric current representing information signal which is fed from a suitable signal processing circuit, not shown, is applied to the winding 2b shown in FIG. 1, thereby causing the auxiliary pole 2 to develop a magnetic field. Under the influence of this field, a strong, sharply defined perpendicular field is produced through the main pole film 1a of the main pole 1, and such perpendicular field causes a reversal in the magnetization of the magnetizable film of the recording medium 4. During reproduction, the perpendicular field produced by the magnetization in the magnetizable film of the recording medium 4 magnetizes the main pole 1, and a change in the magnetization of the main pole 1 produces a change in the magnetization of the auxiliary pole 2, thus inducing an electric current through the winding 2b which may be derived externally and may be electrically processed in a suitable manner to provide a desired electrical signal.
It will be appreciated that a strong, sharply defined perpendicular field must be applied to the recording medium which is sufficient to cause a magnetic saturation thereof in order to achieve the perpendicular recording. In the prior art practice, the material used for the magnetic member 1b of the main pole comprises a block of ferrites such as Mn-Zn or Ni-Zn, which materials however do not exhibit satisfactory magnitude of initial permeability or frequency responses.
Specifically, FIG. 4 graphically shows the frequency responses of Ni-Zn ferrite (curve A) and that of Mn-Zn ferrite (curve B). Thus, the curve A indicates a favorable frequency response up to a high frequency region while the initial permeability .mu.i is low. Conversely, the curve B exhibits a high favorable initial permeability .mu.i while the frequency response thereof does not extend to a high frequency region. As illustrated by these examples, there has been no magnetizable material which satisfies the requirements of the initial permeability and the frequency response. The lack of a combination of these characteristics is attributable to the properties of the ferrite material themselves. As long as such ferrite material is used, it is difficult to provide a perpendicular magnetic recording head which satisfies both requirements.
Accordingly, it is an object of the invention to provide a perpendicular magnetic recording head which eliminates the disadvantages of the prior art by allowing a strong, sharply defined perpendicular field to be applied to a magnetic recording medium.